International Patent Applications WO98/31674 (=U.S. Pat. No. 6,103,718), WO99/31071, WO99/31090, WO99/47505 (=U.S. Pat. No. 6,255,303), WO01/19818, WO01/30766, WO01/30777, WO01/94319, WO02/064584, WO02/085885 and WO02/085906 disclose phthalazinone derivatives having PDE4 inhibitory properties. In the International Patent Application WO94/12461 and in the European Patent Application EP 0 763 5343-aryl-pyridazin-6-one and arylalkyl-diazinone derivatives are described as selective PDE4 inhibitors. International Patent Application WO93/07146 (=U.S. Pat. No. 5,716,954) discloses benzo and pyrido pyridazinone and pyridazinthione compounds with PDE4 inhibiting activity.
In the Journal of Medicinal Chemistry, Vol. 33, No. 6, 1990, pp. 1735–1741 1,4-Bis(3-oxo-2,3-dihydropyridazin-6-yl)benzene derivatives are described as potent phosphodiesterase inhibitors and inodilators. In the Journal of Medicinal Chemistry Vol. 45 No.12, 2002, pp. 2520–2525, 2526–2533 and in Vol. 44, No. 16, 2001, pp. 2511–2522 and pp. 2523–2535 phthalazinone derivatives are described as selective PDE4 inhibitors.